The role of alkali metals as promoters in the methanation and Fischer-Tropsch reaction: An in situ study

Abstract

The effect of alkali metals as catalytic modifiers on the methanation and Fischer-Tropsch reaction over a series of silica-supported Ru catalysts has been studied. When hydrocarbon product distributions were compared at constant CO conversion or at constant CH 4 formation turnover frequencies, rather than at constant temperature, differences in selectivity were small for the series of catalysts studied. The temperature-programmed desorption of CO from these catalysts showed the presence of three desorption maxima, centered at 100, 220, and 450 °C, respectively. The relative intensity of the maximum centered at 220 °C was observed to increase slightly on the catalysts modified by the addition of alkali metals. However, this slight increase in binding energy did not appear to promote the formation of surface carbon. The high-temperature maximum centered at 450 °C was assigned to CO formed by the reaction between surface carbon and H 2O from the support. An “ in situ” infrared study of the CO surface species present on the catalyst surface during reaction showed only one band centered at 2030 cm −1 on supported Ru. This was in contrast to the two bands centered at 2020 and 1950 cm −1, which were observed for the K- and Cs-promoted catalysts. The results of this study suggest a depression in the rate of hydrogenation of the surface carbon species as a result of the addition of the alkali metal modifier. This is explained by invoking site blocking by the alkali metal adatoms rather than by an electronic effect.